The present invention relates generally to a suction connection for a compressor. Specifically, the present invention relates to a suction connection in the evaporator that increases the aerodynamic stability of multiple centrifugal compressors operating in parallel in a refrigeration system.
To obtain increased capacity in a refrigeration system, two centrifugal compressors can be connected in parallel to a common refrigerant circuit. Frequently, for capacity control, one of the compressors is designated as a “lead” compressor and the other compressor is designated as a “lag” compressor. The capacity of the refrigeration system, and of each compressor, can be controlled by the use of adjustable pre-rotation vanes or inlet guide vanes incorporated in or adjacent to the suction inlet of each compressor. Depending on the particular capacity requirements of the system, the pre-rotation vanes of each centrifugal compressor can be positioned to control the flow of refrigerant through the compressors and thereby control the capacity of the system. The positions of the pre-rotation vanes can range from a completely open position to a completely closed position. The pre-rotation vanes for a centrifugal compressor can be positioned in a more open position to increase the flow of refrigerant through the compressor and thereby increase the capacity of the system or the pre-rotation vanes of a centrifugal compressor can be positioned in a more closed position to decrease the flow of refrigerant through the compressor and thereby decrease the capacity of the system.
During operation, a compressor instability or surge condition can occur in a centrifugal compressor, wherein the compressor cannot pump the flow against its discharge pressure. Surge or surging is an unstable condition that may occur when compressors, such as centrifugal compressors, are operated at light loads and high pressure ratios. A high compressor pressure ratio, sometimes called lift or head, may be expressed in a number of fashions. A simplified representation of this compressor pressure ratio is (discharge pressure minus suction pressure (differential pressure or “ΔP”)) divided by suction pressure (“P”), or expressed symbolically, (ΔP)/P. A lower suction pressure will increase the compressor ratio and decrease the stability of a centrifugal compressor. Surge is a transient phenomenon characterized by high frequency oscillations in pressures and flow, and, in some cases, the occurrence of a complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. During a surge condition there can exist a momentary reduction in flow and pressure developed across the compressor. Furthermore, there can be a reduction in the net torque and mechanical power at the compressor driving shaft. In the case where the drive device of the compressor is an electric motor, the oscillations in torque and power caused by a surge condition can result in oscillations in motor current and excessive electrical power consumption.
In dual compressor applications, the occurrence of a surge or lack of pumping condition on one compressor results in the other compressor having an increase in refrigerant flow. This increase in refrigerant flow to the non-surging compressor makes it more difficult for the surging compressor to recover from its instability. Axial gas flow within the evaporator to the stable compressor will pass over a suction opening of the unstable compressor, thereby lowering the pressure at the unstable compressor suction connection which further contributes to instability. Several different techniques have been used to limit the potential aerodynamic impact one compressor may have upon the other compressor in a dual compressor system. Some chiller systems with two compressors utilize two completely separate refrigerant circuits to avoid the problem of one compressor aerodynamically impacting the other compressor. Other dual compressor chiller systems which use a common refrigerant circuit have a baffle in the gas plenum space of the evaporator between the suction connections of the compressors to reduce the aerodynamic impact of one compressor upon the other compressor. In this type of system each of the two suction connections are typically located approximately one quarter of the evaporator shell's length from the ends of the evaporator shell, because of the baffle or partition bisecting the evaporator shell into substantially equal halves. Both of these solutions have several drawbacks including a more complicated and expensive implementation of the evaporator. A completely separated evaporator shell would result in less heat exchanger surface being available during single compressor operation, and therefore would provide less effective heat transfer and reduced performance. Flooded shell and tube evaporators boil refrigerant liquid on the shell side to cool water flowing through the tubes. The refrigerant gas flow evaporating off the liquid surrounding the tubes will carry some of the liquid along with the gas. Evaporator heat exchangers typically use baffle passages or mesh pad eliminators to remove the liquid droplets from the gas before entering the compressor suction. If the vapor space above the baffle or mesh pad is separated into halves, as in some systems, the boiling activity in single compressor operation is concentrated in one half of the evaporator using one half of the mesh pads. This provides less effective vapor separation than if the entire baffle or mesh pad section were utilized.
Therefore, what is needed is a simple and economical suction connection for use in a dual compressor refrigeration system that can increase pressure at the suction connection to encourage the flow of refrigerant vapor into a surging compressor to thereby enhance the ability of the surging compressor to recover from its instability.